25 April 2007, 14:02 in

Interlacing

Recently I was playing around on my dad’s video editing computer and was trying to render to both DivX and XviD from a high-definition Sony Vegas 7 project. I was having trouble with ugly horizontal lines appearing whenever there was motion in the image. Turns out it was an issue with interlacing.

I vaguely knew about interlacing but decided to do a bit of research to find out more. I found a good site called 100fps.com, and the Wikipedia entry is also very good. I will attempt to explain in my own words…

Video recordings can be categorized as being either progressive (non-interlaced) or interlaced. Interlacing is a technique that allows for a video signal to have smoother motion, at the sacrifice of some still image quality, without requiring any extra bandwidth.

With progressive scan, an image is created line by line. Let’s say we have a 720p signal at 25fps. Then, each and every frame, 720 horizontal lines are drawn in order to create the image. This would be done 25 times per second to give a frame rate of 25 fps. Simple, makes sense.

Interlacing works a bit differently. Say we have an interlaced video signal at a certain resolution (number of lines) and frame rate, say 1080i at 25 fps. This signal actually consist of 50 fields per second, where each field has half the resolution of the output image so 540 lines. So a single frame of 1080i video contains 1080 lines, but the image is made up from two fields of 540 lines each; the odd lines come from one field and the even lines from the other field. So a frame consists of two captures (fields) from two moments in time.

If we simply combined two consecutive fields from an interlaced video signal to get a single full resolution frame then, because these fields represent slightly different moments in time, we get some visual artifacts wherever there is movement.

So what is the point of interlacing then? From 100fps.com:

Movies with 25 non-interlaced (=progressive) frames per second don’t look very fluid. If you watched a football game with 25 progressive fps it would look as if the ball isn’t flying fluidly thru the air. With 50 fields per second which are then combined to 25 frames per second this looks much better.

So interlacing is in fact a clever way to compress a movie when one cannot use digital compression methods. Interlacing reduces the bandwidth (= storage space nowadays) by half, without losing vertical resolution in quiet areas (in motion areas you don’t notice very much anyway, because it’s moving 50 times per second). So interlacing is a way to display the nonmoving parts with full resolution and the moving parts with half resolution, but fluidly. It’s a very clever way to cut bandwidth without sacrificing much quality.

So, when recording video, there is the option of recording non-interlaced for higher vertical resolution, or interlacing for smoother motion. If had we unlimited bandwidth, we could simply use a high resolution to give good picture quality, and a high frame rate to achieve smooth motion, and not worry about the extra complexity of interlacing.

From Wikipedia:

The perceived vertical resolution of an interlaced image is usually equivalent to multiplying the active lines by about 0.6. This means that, when viewing progressive sourced material, a progressive display will show a more detailed image, when compared to an interlaced one, even if both have exactly the same display resolution.

To display interlaced video, the fields must be somehow turned into frames. This process is known as deinterlacing. Various methods exist, including blend (also known as average or combine fields), weave (do nothing), area based, motion blur, discard (discard every second field, so you show only the odd or even fields and resize them to the full frame size), bob (also known as weave; the fields are shown one at a time, scaled up to the frame size, but at twice the frame rate), and progressive scan.

Hopefully this is of some help if you are confused by the terms interlacing and progressive scan.

  1. They’re pushing for 120 fps TV’s now to take advantage of high resolution 60 fps media interlaced or real 120 fps media to avoid motion issues on 1080p media. Just when you thought it was safe to get into HDTV…


    — PC    Apr 27, 11:45 am    #
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